Saturday, 14 December 2013

Solar updraft towers a renewable-energy power plant

Solar updraft towers a renewable-energy power plant

Solar
updraft towers, also known as solar chimneys, are a relatively new concept
of renewable energy power plant that may become widespread in the future.
They would combine three old and proven technologies: the chimney effect,
the greenhouse effect, and the wind turbine. Air would be heated by
sunshine and contained in a very large greenhouse-like structure around
the base of a tall chimney; the resulting convection would cause air
to rise up the updraft tower. This airflow would drive turbines, producing
electricity.A first
research prototype operated in Spain in the 1980s. Many modelling studies
have since been published as to their optimisation, scale and economic
feasibility. Some proposals have involved mega-structures reaching up
to a kilometre in height.* A small operating plant is reported to have been built in Jinshawan,
China, as of 2011.The solar updraft tower (SUT) is a renewable-energypower plant for generating electricity from solar power.
Sunshine heats the air beneath a very wide greenhouse-like roofed
collector structure surrounding the central base of a very tall chimney tower. The resulting convection causes a hot air updraft in the tower by the chimney effect. This airflow drives wind turbines placed in the chimney updraft or around the chimney base to produce electricity.
Plans for scaled-up versions of demonstration models will allow
significant power generation, and may allow development of other
applications, such as water extraction or distillation, and agriculture
or horticulture.As a solar chimney power plant (SCPP) proposal for electrical power
generation, commercial investment is discouraged by the high initial
cost of building a very large novel structure, and by the risk of
investment in a feasible but unproven application of even proven
component technology for long-term returns on investment—especially when
compared to the proven and demonstrated greater short-term returns on
lesser investment in coal-fired or nuclear power plants. Likewise, the
benefits of 'clean' or solar power technologies are shared, and the
widely shared harmful pollution of existing power generation
technologies is not applied as a cost for private commercial investment.
This is a well-described economic trade-off between private benefit and shared cost,
versus shared benefit and private cost. If it is in the public
interest, then some form of public investment or subsidy to share cost
and risk will be required to demonstrate SCPP feasibility at scale.Power output depends primarily on two factors: collector area and
chimney height. A larger area collects and warms a greater volume of air
to flow up the chimney; collector areas as large as 7 kilometres
(4.3 mi) in diameter have been discussed. A larger chimney height
increases the pressure difference via the stack effect;
chimneys as tall as 1,000 metres (3,281 ft) have been discussed. Due to
variations in design, climate, local geography and latitude, a
standardised model for comparisons between design features and outputs
is needed and proposedHeat can be stored inside the collector area. The ground beneath the solar collector, water in bags or tubes, or a saltwater thermal sink
in the collector could add thermal capacity and inertia to the
collector. Humidity of the updraft and condensation in the chimney could
increase the energy flux of the system.Turbines
with a horizontal axis can be installed in a ring around the base of
the tower, as once planned for an Australian project and seen in the
diagram above; or—as in the prototype in Spain—a single vertical axis
turbine can be installed inside the chimney.Carbon dioxide is emitted only negligiblyas part of operations. Manufacturing and construction require substantial power, particularly to produce cement. Net energy payback is estimated to be 2–3 years.Since solar collectors occupy significant amounts of land, deserts and other low-value sites are most likely.A small-scale solar updraft tower may be an attractive option for remote regions in developing countries.The relatively low-tech approach could allow local resources and labour to be used for construction and maintenance.Locating a tower at high latitudes could produce up to 85 per cent of
the output of a similar plant located closer to the equator, if the
collection area is sloped significantly toward the equator. The sloped
collector field is built on suitable mountainsides, which also functions
as a chimney. A short vertical chimney on the mountaintop to
accommodate the vertical axis air turbine. The results showed that solar
chimney power plants at high latitudes may have satisfactory thermal
performance.Solar updraft towers can be combined with other technologies to increase output. Solar thermal collectors or photovoltaics can be arranged inside the collector greenhouse. This could further be combined with agriculture

Efficiency

The solar updraft tower has a power conversion rate considerably lower than many other designs in the (high temperature) solar thermal group of collectors. The low conversion rate is balanced to some extent by the lower cost per square metre of solar collectionModel calculations estimate that a 100 MW plant would require a 1,000
m tower and a greenhouse of 20 square kilometres (7.7 sq mi). A 200 MW
tower with the same tower would require a collector 7 kilometres in
diameter (total area of about 38 km²).
One 200MW power station will provide enough electricity for around
200,000 typical households and will abate over 900,000 tons of
greenhouse producing gases from entering the environment annually. The
collector area is expected to extract about 0.5 percent, or 5 W/m² of
1 kW/m², of the solar energy that falls upon it. Concentrating thermal (CSP) or photovoltaic (CPV) solar power plants range between 20% to 31.25% efficiency (dish Stirling).
Overall CSP/CPV efficiency is reduced because collectors do not cover
the entire footprint. Without further tests, the accuracy of these
calculations is uncertain.The performance of an updraft tower may be degraded by factors such as atmospheric winds,by drag induced by the bracings used for supporting the chimney,and by reflection off the top of the greenhouse canopy.